Electrochemical cells, especially high energy density cells such as those in which lithium is an active material, are subject to leakage or rupture as a result of various abusive treatment, which in turn, can cause damage to a device which is powered by the cell or to the surrounding environment. Such cells are sealed to prevent egress of electrolyte solvent and ingress of moisture from the exterior environment. When a cell is overcharged, self-heating occurs during the charging procedure. Examples of abusive treatment include charging at too rapid a rate, overcharging above a specified maximum voltage or discharging below a specified minimum cell voltage which can lead to an increase in temperature and/or pressure. In the case of cells utilizing lithium, should the temperature exceed a certain point an exothermic reaction begins which leads to overheating and causes a build-up in pressure which may result in expulsion of electrolyte from the cell.
It is known for commercially available cylindrical lithium ion electrochemical cells to incorporate a pressure driven current interrupt device to terminate current flow under adverse charge or discharge conditions. Typically, a thin metallic diaphragm, e.g., formed of aluminum, mounted in a header of the cell and exposed to pressure generated in the cell is attached to a pressure plate at the central axis, as by laser or ultrasonic welding, with an electrical insulator sandwiched therebetween. As pressure increases stress is created at the welded interface with the weld joint fracturing at a selected pressure range allowing the diaphragm to translate away from the plate to permanently open the electric circuit and terminate current flow. In some designs, the diaphragm is formed with a reduced thickness portion, e.g., a C-shaped groove extending along the peripheral edge of the diaphragm so that the diaphragm ruptures at a calibrated predetermined pressure to allow controlled venting of pressurized electrolyte. A current interrupt device of this type is shown in U.S. Pat. No. 5,691,073.
The above described end cap and current interrupt device assembly can be used conveniently in cylindrical electrochemical cells typically having diameters of 17 mm or more. However, as cell size is reduced the available membrane surface used to drive the pressure current interrupt device is reduced so that with prismatic cells having a thickness of only 10 mm, 8 mm or 6 mm, for example, the diameter of the working portion of the diaphragm is too small to provide sufficient translation to fracture the welded joint with the pressure plate so that a different operational mechanism is needed to provide the current interrupt function. As a result of the size limitations of such prismatic cells, the prevalent header related safety feature currently in commercial use is a pressure vent which allows controlled release of pressurized electrolyte in order to prevent cell explosion.